The invention relates to a seal arrangement for the sealing of a shaft, guided through the bore of a housing wall, against the egress of a liquid medium when the shaft turns in a given direction or when it is at rest.
The prior art includes seals which have one or more annular sealing lips made of elastic material, especially rubber, which are pressed against the shaft by an edge prestressed radially inward by spring force. In simple applications, the radially inwardly directed force can be produced by the elastic property of the sealing lip itself or by an endless spiral spring surrounding the sealing lip. Seals of this type have the benfit of a low manufacturing cost and space requirement and therefore are utilized widely in practice. But their utilization is restricted, on one hand, by the temperature that occurs at the sealing lip as a result of the elevated temperature of the sealing lip due to frictional heat, and, on the other hand, by the wearing and/or a lifting off of the sealing lip when the shaft undergoes large dynamic motions.
A low contact force of the sealing lip is sufficient to seal the shaft reliably when it is at rest. If the same sure sealing action is to be achieved for rotating shafts also, then the contact force must be raised substantially in order to prevent the sealing edge from lifting and a leak flow developing due to an eccentricity or vibrations of the shaft. Accordingly, at high peripheral speeds, a considerable development of heat occurs at the sealing edge which presses practically linearly on the shaft. If the temperature of the liquid medium to be sealed or of the shaft is relatively high, then, depending on the maximum thermal load capacity of the elastic material of the sealing lip, the limit of utilizability of the seal is soon reached. The lip seal is sensitive to even the smallest damages, e.g. scratches, of the contact surface of the shaft interacting with it. This effect occurs when the shaft is rotating, but practically not at all when it is at rest.
Other known seals have an annular member, arranged in the housing bore to be sealed off, which with its cylindrical inner surface surrounds the shaft with a slight radial clearance. Hollowed out in the inner surface are thread-shaped grooves whose thread direction chosen in the sense of a return of the escaping liquid medium into the space to be sealed off. For such types of seals, there is no restriction for the range of high peripheral speeds. But since the return action decreases as the rotational speed of the shaft falls and becomes equal to zero when it is at rest, it is necessary to form the grooves with a small cross-sectional area and to select a square-threaded variation in order that the leak flow remain small under the indicated operating conditions. But, due to these restrictions, it is not possible to form the grooves for an optimal return action and thereby make them suitable for the sealing of a liquid medium which under certain circumstances may even be under pressure.
The return action during rotation and also the limitation of the leakage while at rest presupposes the maintenance of a minimum radial clearance between the shaft and the stationary annular member. Accordingly, the utilization of such known seals is limited to exactly concentrically running shafts with exact machining of the shaft's surface. But these conditions can not always be respected in practice, e.g. for crankshaft seals, particularly because of bearing wear.
An important object of the invention is to create a seal arrangement which reliably suppresses a leak flow when a shaft is rotating or at rest and whose utilization limits with respect to the shaft's rotational speed and particularly with respect to the shaft's peripheral speed are raised and which places low requirements on the concentric running of the shaft.
According to the invention, this problem is solved by the combination of an annular sealing lip made of elastic material which is fastened in a sealing manner to the wall or the shaft and which with its sealing edge is pressed by spring force against an annular surface, in fixed relation to the shaft or the wall, respectively, and coaxial with the shaft, with a force that with consideration to the aforesaid installation and operating conditions reliably prevents a leak flow when the shaft is at rest but that, at least in the heated state of the sealing lip to be anticipated under normal operating conditions, permits a leak flow at full rotational speed of the shaft, together with a return mechanism, connected in series with said sealing lip with respect to the leak flow, consisting of a hollow body which is supported on the wall in a nonrotatable sealing manner and displaceable in the radial direction against spring action and which is made of a material having good antifriction properties, such as polytetrafluorethylene, graphite, metallic powder, impregnated asbestos or a mixture of such materials, and which with its cylindrical bore surrounds, with slight radial clearance or under sliding contact, a longitudinal section of the shaft or a sleeve fastened to the latter in a sealing manner, with thread-shaped return grooves being cut in the bore of the hollow body or in the longitudinal section of the shaft or the sleeve surrounded by the latter hollow body.
By means of the above described construction of the seal arrangement, the return mechanism, and particularly the construction of the thread-shaped grooves, can be designed for an optimal return effect with a rotating shaft because a leak flow with a resting shaft is hindered by the sealing lip which, for its part, need exert no sealing action when the shaft is turning and therefore can make do with a substantially lower contact force than would have to be provided according to the usual dimensioning rules. The hollow member having the return grooves can be adapted to extensions of the shaft so that, when determining the size of the radial clearance between its bore and the shaft surface surrounded thereby, only production tolerances need be considered, but not the accuracy of the shaft bearing or the magnification of the bearing play during operation. The possible reduction of the radial clearance of the return mechanism leads to an improved return effect.
According to the invention, for the reliable prevention of a leak flow with a stationary shaft the sharp sealing edge of the cold sealing lip should be applied to the associated annular surface with a contact force of at least 50 grams per centimeter of edge-circumference length, with materials ordinarily utilized for lip seals and at least 25 grams per centimeter of edge circumference length for extremely soft materials, with the mentioned values being valid without influence by a possible present centrifugal force. On the other hand, this contact force relative to the cold state should be less than 150 grams per centimeter of edge-circumference length, for extremely soft materials 75 grams per centimeter of edge-circumference length in order that a leak flow can occur under normal operating conditions of the shaft. If the sealing lip is integral with the shaft and during rotation is subjected to the action of the centrifugal force in the direction of a reduction of the originally present contact force, then a pressureless application of the sealing lip to the annular surface integral with the wall or a lifting of said annular surface can be achieved, with any wear of the sealing edge during operation being eliminated.
It is essential for the determination of the magnitude of the contact force that a reliable sealing should be achieved by the sealing lip when the shaft is at rest, allowing for all operating circumstances, whereas a leak flow should occur at full rotational speed of the shaft. It can thereby be achieved that the sealing lip is operated not in the usually present state of mixed friction, but rather of liquid friction. In conjunction with the lower contact force and improved lubrication, the frictional wear and frictional heat and therewith the temperature rise of the sealing edge relative to the temperature of the medium to be sealed can thus be sharply reduced. Considering the fact that under ordinary operating temperatures of 120.degree.C to 130.degree.C, such as are observable in crankshaft seals of internal combustion engines, a drop of 10.degree.C in the temperature at the sealing edge already increases the service life of the sealing lip by 30-50%, there results a substantially longer service life of the sealing lip. But with the seal arrangement according to the invention, a sealing lip dimensioned with such a low contact force can also be utilized for shaft peripheral speeds that heretofore were ruled out for known sealing lips.
According to another feature of the invention, the sealing lip forms a ready-to-install unit together with the return mechanism. This unit preferably also contains a dust guard which is provided on the suction side of the return mechanism and axially before the hollow body which forms an essential component thereof and which surrounds the shaft annularly, with the dust guard being pressed against the shaft with minimal pressure or being adjusted to it with slight radial clearance. The dust guard prevents dust particles or small particles from being sucked in by the suction action of the return mechanism, reaching the sealing edge and damaging the latter by wear. The dust guard consists preferably of an air-permeable easily deformable filter material, particularly a nonwoven fabric made of synthetic and/or natural fibers.
Other objects, features and advantages of the invention are given in the following detailed description of preferred embodiments, taken in connection with the accompanying drawing in which: